In order to obtain the novolacs above-referred to with minimal contents of oligomeric molecules it is necessary not only to employ an excess of the methylol-reactive phenol but also to hold down the reaction temperature. The methylol groups are reactive not only with phenol ring hydrogens (the desired reaction for purposes of this application) but also with each other--which results in oligomerization. Unfortunately, the acids which catalyze the desired reaction tend to more effectively catalyze the undesired reaction as the temperature goes up.
Once initiated, the desired condensation is exothermic. On a small reaction scale, the evolved heat can be removed rapidly enough by heat transfer so that the reaction temperature can be held down. However, this is not feasible on a large scale (due to the viscosity of the reaction mixture and its relatively low thermal conductivity). This is largely true even if--in addition to heat exchange with a coolant--removal of heat is accomplished by boil-off of the water (and methanol) formed in the desired (and undesired) reactions.
The reaction of course can be moderated with a solvent, which may also improve sensible heat transfer by thinning the reaction mixture. However, this results in a lower average reaction rate, reduces the efficiency of reactor volume utilization, requires the use of more catalyst and necessitates subsequent removal of the solvent.
It was found that the rate of heat evolution could be reduced by incremental addition of a preformed mixture, of part of the methylol-reactive phenols with all of the polymethylol diphenol, to a mixture of the rest of the phenol and the catalyst, at the desired reaction temperature. However, even this resulted in an undesirably high content of oligomers in the novolac product. Apparently, the rate of self-reaction of methylol groups is much faster than the rate of mixing which can be achieved economically.